Ti3AlC2 MXene nanosheets as a novel corrosion inhibitor for carbon steel in 0.5 M sulfuric acid solution

Herein we report that Ti3AlC2 MXene nanosheets were identified as a highly effective cathodic protection corrosion inhibitor for carbon steel in hydrochloric acid solution. Ti3AlC2 Mxene nanosheets form a stable inhibition layer on metal surfaces due to their high adsorption capacity and act as a barrier or protective film to prevent attacks from corrosive substances and thus lead to an extended metal service life.


Materials
Ti 3 AlC 2 nanosheets were synthesized by etching Ti 3 C 2 phase with LiF/HCl as described earlier.The process involves adding Ti 3 C 2 phase to 10 mL of 9 M HCl containing 1.0 g of LiF and 0.16 g of AlCl 3 , followed by 3 days of etching at room temperature.The precipitate obtained was washed several times with deionized water until neutral pH was reached and dispersed in TMAOH (25% wt in water) for 3 days with stirring.After centrifugation and repeated washing with deionized water and ethanol, the collected precipitates were re-dispersed in water using sonication in a water bath.Finally, an aqueous solution of Ti 3 AlC 2 nanosheets were obtained from the supernatant after centrifugation at 7000 rpm.The morphology and microstructure of Ti 3 AlC 2 nanosheets were investigated by scanning electron microscope (SEM, SU1510, Japan).The Zeta potential technique was carried out to determine the stability and charge of nanoparticles.
The powder form of Ti 3 AlC 2 (200-mesh) and TMAOH solution with a concentration of 25% in water were acquired from Shanghai Macklin Biochemical Co., Ltd.High-purity HCl solution (> 98%) with a concentration of 36% (w/w) and AlCl 3 were obtained from Sinopharm Chemical Reagent Co., Ltd.The study used ultrapure water with a resistivity of 18.2 MΩ•cm sourced from Milli-Q Biocel.Q235 carbon steel with 1 × 1× 0.5 mm was well polished using SiC paper with 180 to 1500 mesh before each weight and electrochemical tests.

Measurement of corrosion rate and corrosion inhibition efficiency
The polished Q235 carbon steel sample was weighed at least three times by analytical balance and then averaged before and after being soaked in H 2 SO 4 solution containing different concentrations of MXene corrosion inhibitor at room temperature.
The surface of sample was cleaned using the pickling solution containing 1% methenamine.Corrosion rate and corrosion inhibition efficiency (η) can be calculated as follows: (1) (2) Where, w 0 and w are the mass loss of Q235 carbon steel without and with MXene inhibitor, v 0 and v are the corresponding average corrosion rate without and with corrosion inhibitor, t is the immersion time, and S t is the exposed electrode area.

Measurement of electrochemical impedance spectra (EIS) and potentiodynamic polarization curves (PDP)
The CS350 electrochemical workstation (Corrtest, China) with a three-electrode configuration was used to measure electrochemical impedance spectra (EIS) and potentiodynamic polarization curves (PDP) for evaluation of anti-corrosion performance of Ti 3 AlC 2 MXene nanosheets.The encapsulated carbon steel with exposed area 1 cm 2 was working electrode, saturated calomel electrode (SCE) acted as reference electrode and platinum tablet with exposed area 2 cm 2 acted as counter electrode, respectively.PDP tests were carried out with a scanning rate of 0.5 mV/s ranging from -0.15 to +0.15 V versus open circuit potential.Tafel extrapolation was uniformly employed to fit the polarization data.The region on the polarization curve that conformed to the Tafel relationship was selected, namely, the area where a linear relationship existed between current density and electrode potential.Within the designated Tafel region, a linear regression method was applied to fit the data.The slope of the fitted line represented the Tafel slope, while the intercept could be used to calculate the corrosion current density.The physicochemical meanings of each parameter for the polarization curve were as follows: I 0 was corrosion current density and E corr was self-corrosion potential.η PDP is the anti-corrosion effectiveness from PDP method could be calculated as Eq.(3).

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(3) where and were the corrosion current density of Q235 carbon steel without  0  ' 0 and with corrosion inhibitor, respectively.
EIS tests employed a sinusoidal wave disturbance of 10 mV amplitude at frequency range of 10 4 to 10 -2 Hz.The physical and chemical meanings of each parameter in the equivalent circuit in the following table were: R s was solution resistance.CPE f and CPE dl were the double layer capacitor of film and electric double layer, respectively.
R f was the resistance of film and R ct was the resistance due to charge Transfer.The η EIS was calculated as follows: (4) where R ct and R ct 0 respectively represented the charge transfer resistance in the presence and absence of MXene corrosion inhibitor, respectively.
The variance (σ) was displayed to illustrate the accuracy of fitting process.
According to the principle of a parallel-plate capacitor, capacitance was proportional to the plate area and inversely proportional to the thickness of the insulator.This relationship was expressed as Eq. ( 5): where ε was the dielectric constant, d was the thickness of the insulator, and S represented the plate area.
The SVET measurement technique was employed to investigate the localized corrosion behavior using the VersaSCAN microelectrochemical system.The vibration amplitude was 30 μm and swept area was set as 6 × 6 mm 2 .
Table S4 Parameters for EIS measurements with different concentrations of MXene nanosheets.